1. Field of the Invention
The present invention concerns a controllable electrochromic indicator device consisting of an electrochromic layer and an adjacent charge-carrier-passing insulating layer located between two electrodes with at least one of the two electrodes attached to a supporting plate and at least one of the electrodes being transparent.
2. Description of the Prior Art
Such indicator devices are already known from U.S. Pat. No. 3,521,941. A particular embodiment is shown in FIG. 1. On a supporting plate 1, e.g., of glass or another transparent material, there are deposited in succession thin layers of tin oxide (SnO.sub.2) as a transparent electrode 2, molybdenum oxide (MoO.sub.3) as an electrochromic substance 3, silicon oxide (SiO) as a charge-carrier-passing insulator 4 and gold (Au) as the second electrode 5. A voltage AB of about 6V with the proper polarity applied to the electrodes 2, 5, the thickness of the charge-carrier-passing insulating layer 4 being about 0.2 .mu. and that of the electrochromic layer being about 1 .mu., produces a blue coloration of the previously almost colorless indicator device after about 30 seconds. The color fades away about 15 seconds after reversal of the polarity of the voltage.
The charge carrier transmitting insulation layer 4 on the one hand is to prevent a short circuit between electrodes 2 and 5, but on the other hand it is also to supply a sufficient flow of charge carriers to the electrochromic layer 3. A satisfactory number of charge carriers pass through the insulating layer 4 only at very high field strengths, so as to produce coloration of the electrochromic layer 3. Since the speed of coloration and thus the response time of the device is determined by the number of charge carriers reaching the electrochromic layer per unit time and the field strength cannot be made arbitrarily high, such indicator devices exhibit a slow response and must always be operated at high field strengths, i.e., at relatively high voltages and very small thicknesses of the charge-carrier-transmitting layer 4. Moreover, electrolytic deposits form on the electrodes of such indicator devices in the course of time, thus shortening the lifetime of the electrochromic cell.